#6 PERFORMANCE: use single quotes instead of double quotes

[HM2K]

Wrap your string in single quotes (’) instead of double quotes (”) is faster because PHP searches for variables inside “…” and not in ‘…’, use this when you’re not using variables you need evaluating in your string. [http://spindrop.us/2007/03/03/php-double-versus-single-quotes/]

From: http://www.hm2k.com/posts/50-php-optimisation-tips-revisited